The present invention relates to silicon carbide (SiC) based semiconductor devices that operate at high voltage and thus in the presence of, or otherwise generate or experience, high electric fields. Such devices typically include, but are not necessarily limited to Schottky (rectifying) diodes, metal-oxide semiconductor field-effect transistors (MOSFETs); insulated gate bipolar transistors (IGBTs); PIN diodes; bipolar junction transistors (BJTs). For example (but not as a limitation), SiC-based power devices are advantageous for (switching) power supplies, motor control, power conditioning, hybrid vehicle technology, safety equipment, and power storage.
For electronic power devices, silicon carbide offers a number of physical, chemical and electronic advantages. Physically, the material is very hard and has an extremely high melting point, giving it robust physical characteristics. Chemically, silicon carbide is highly resistant to chemical attack and thus offers chemical stability as well as thermal stability. Perhaps most importantly, however, silicon carbide has excellent electronic properties, including high breakdown field, a relatively wide band gap (about 2.9 eV at room temperature for the 6H polytype), high saturated electron drift velocity, giving it significant advantages with respect to high power operation, high temperature operation, radiation hardness, and absorption and emission of high energy photons in the blue, violet, and ultraviolet regions of the spectrum.
For power applications, silicon carbide's wide bandgap results in a high impact ionization energy. In turn, this allows SiC to experience relatively high electric fields without avalanche multiplication of ionized carriers. By way of comparison, silicon carbide's electric field capacity is about ten times as great as that of silicon.
Because the active regions of these devices experience or generate such high electric fields, the devices typically must include some sort of termination structure to lessen the effects of the field (“field crowding”) at the edge of the device. In common examples, the termination structure includes implanted regions in the silicon carbide adjacent the active region. Because the surface of the device must also be terminated, some sort of passivation structure is typically added to this surface. In most cases, the surface passivation structure can include a polymer (frequently polyimide) or a dielectric passivation such as silicon oxide, silicon nitride, or some combination of these, including non-stoichiometric oxides and non-stoichiometric nitrides (i.e., other than SiO2 and Si3N4).
As SiC-based devices capable of handling higher voltages, and thus higher electric fields, have continued to be developed, it has been unexpectedly discovered that silicon carbide, which is normally chemically resistant to attack at temperatures below about 1500° C., will nevertheless oxidize at much lower temperatures in the presence of the high electric fields associated with these types of power electronic devices. In particular, this oxidation is present in devices where significant transient current passes through the device such as in switching power devices. As best understood, this appears to be a otherwise conventional oxidation reduction reaction in which the presence of the electrons applied at the higher fields and at relatively higher frequencies encourages the oxidation to take place.
This unexpected and undesired oxidation of silicon carbide can take place at relatively modest operating temperatures; i.e., as low as 125° C. rather than the normally much higher temperatures required before silicon carbide will participate in a chemical reaction.
As the resulting undesired oxide grows, it expands and tends to lift the passivation layer away from the device, eventually degrading or eliminating its performance characteristics.
Conventional oxide passivation techniques also tend to exhibit drift at high electric fields. As set forth in the '378 application, at least some of this drift is attributed to the presence of hydrogen (present as hydrogen ions), which tend to drift toward the negative electrode, resulting in charge accumulation that reduces the blocking capacity of the device and the device's overall capabilities. The presence of hydrogen typically results from the use of plasma enhanced chemical vapor deposition (PECVD) to form the passivation structures, because many of the oxide precursors used in PECVD include hydrogen.
The problems discovered in this regard have been observed at field strengths as low as 250 kilovolts per centimeter (kV/cm), and are definitely evident at 500 kV/cm and above. Many SiC-based power devices experience fields as high as about 1.5 megavolts per centimeter (MV/cm).
Accordingly, devices that will take full advantage of the field strength characteristics of silicon carbide require passivation structures that can withstand such field strengths without undesired electronic behavior such as drift and without corrosive oxidation-reduction reactions such as the oxidation of silicon carbide to some stoichiometric or non-stoichiometric silicon oxide.